Method for the manufacture of single crystals

ABSTRACT

The molten substance is poured into a cluster of moulds having their lower ends connected to a feeding funnel. The cluster is slowly lifted out of a liquid whose temperature is higher than the melting point: as a result the isothermal planes in the substance are strictly horizontal, undistorted and at the same level in all moulds.

United States Patent [72] inventors FrancisBusquet Saint-Martin-Le-Vinoux; Francis Forrat, Seyssinet-Pariset, both of, France [21] Appl. No. 848,974

[22] Filed Aug. 11, 1969 [45] Patented June 15, 1971 [73] Assignee Commissariat A LEnergie Atomique Paris, France [32] Priority Aug. 23, 1968 [33] France [54] METHOD FOR THE MANUFACTURE OF SINGLE CRYSTALS 2 Claims, 1 Drawing Fig.

[52] U.S. Cl 164/60,

164/122, 164/338, 164/361 [51] Int. Cl 822d 25/06 [50] Field of Search 164/60, 122, 125, 127, 338, 353, 361

[56] References Cited UNITED STATES PATENTS 1,793,672 2/1931 Bridgman 23/301 2,594,998 4/1952 Rocco 164/60 3,260,505 7/1966 Ver Snyder 253/77 3,441,078 4/1969 Chandley 164/53 3,532,155 10/1970 Kane et al 164/60 Primary Examiner-J. Spencer Overholser Assistant Examiner-John E. Roethel Attorney-Cain eron, Kerkam & Sutton ABSTRACT: The molten substance is poured into a cluster of moulds having their lower ends connected to a feeding funnel.

The cluster is slowly lifted out of a liquid whose temperature is higher than the melting point: as a result the isothermal planes in the substance are strictly horizontal, undistorted and at the same level in all moulds.

METHOD FOR THE MANUFACTURE OF SINGLE CRYSTALS This invention relates to a method for the manufacture of single crystals, particularly of metals and alloys, and a system for carrying out the method.

Among present methods for manufacturing single crystals of metal, one of the most commonly used because of its sim plicity is the Bridgman method. According to this method, the metal is melted in a crucible with a conical end, the temperature in the crucible is stabilized to a value near its melting point (value corrected by a difference corresponding to the superfusion factor), and a negative heat gradient of a few degrees per centimeter is moved along a crucible at a low speed from the tip of the cone. At a suitable speed a crystal germinatcs at the tip of the cone and grows only in accordance with the movement of the heat gradient, eliminating superfi- -cial twin crystals, which are thrown to the edge of the crucible, at least if certain conditions are satisfied. in particular, the growth front of the single crystal must be convex. This means that most of the heat losses must be produced by conduction through the already formed crystal and not by means of the wall at a level at which the central portion of the body is not yet crystallized. There are various empirical formulas giving the limiting values to be observed, in dependence in particular on the thermal conductivity of the crystal and of the liquid.

Although this method generally gives satisfactory results it has its limitations. In particular, it is very unsuitable for the simultaneous manufacture of a group of identical single crystals in a cluster of crucibles moved en bloc. Tests carried out with a cluster of crucibles formed by holes closed at one end, terminating in a tip at the bottom and formed in a block of graphite or ceramic material heated by induction have been very disappointing. One of the reasons is the low thermal conductivity of the materials that may be used to form the block. It is very difficult to obtain a substantially flat and horizontal isotherm and therefore crystallization at the same level in all the holes and satisfactory homogeneity of the crystals.

This invention aims to provide a method and system that satisfies the requirements of the art better than hitherto, particularly by enabling substantially identical single crystals to be manufactured simultaneously.

For this purpose, the invention provides a method for the manufacture of single crystals from a substance by vertical movement of a solidification front along a previously melted mass of the substance, the method being characterized in that the melted substance is subjected to hydrostatic pressure in a mould of high thermal conductivity immersed in a bath of molten material kept at a temperature slightly higher than the melting point or liquidus of the substance and in that the mould, the top end of which ends in a point, is taken gradually out of the bath.

The invention also provides a system for the manufacture of single crystals of a substance, the system comprising a cluster for receiving the molten substance formed by a plurality of moulds closed at the top and by a channel communicating with the bottoms of the moulds, a tank provided with heating means for bringing a liquid material in the tank to a temperature higher than the melting point of the substance and means for raising the cluster from a bottom position in which the moulds are totally immersed in the material in the tank.

Among the advantages of the invention, the following may in particular be mentioned. The method completely satisfies the theoretical conditions of simultaneous crystallization in several moulds taken at the same time out of the bath of molten material, since the isotherm is fixed for all the moulds by the bath, at a temperature that is uniform throughout its mass. The use of moulds of the carapace" type enables single crystals of irregular shape to be manufactured directly in the dimensions required for use, the maintenance of a hydrostatic pressure ensuring that the crystal is kept in position while growing and is in close contact with the mould.

The invention will be more readily understood by reading the following description of the system forming an embodiment of the invention, given by way of nonlimitative example. The description refers to the accompanying drawing, the single FIGURE of which shows the system in elevation and in partial section along a vertical plane passing through its axis.

The system shown in FIG. 1 comprises a metal vessel 10 with a leakproof lid 12. One or several jacks l4 enable the lid 12 to be raised or lowered. Seals (not shown) are interposed between the lid and the top edge of the vessel.

The inside of the vessel being raised during operation to a high temperature, the wall of the vessel and, if required, the lid are provided with a lining l6 bounding with the outer wall of the vessel an annular gap 18 for a liquid coolant to flow through (generally water). Means (not shown) are provided for producing within the vessel a vacuum of at least 10 mm. Hg, to prevent the oxidation of the material for forming the single crystals.

The vessel 10 contains a melting furnace 20 with heating by induction with a high frequency or average frequency current, to melt the substance of which the single crystals are to be formed, and a crystallization furnace 22, which also has a high frequency heating system.

The melting furnace 20 is formed by a crucible 24 carried by the wall of the vessel by means of pivots (not shown), which enable it to be tilted and to :move from the position shown in solid lines to the one shown in broken lines in the drawing. The tilting is activated by a handwheel secured to one of the pivots through the vessel and not visible in the drawing. The crucible 24 has a high frequency coil 26 connected to a generator whose power is sufficient to melt a charge of the substance to be treated and placed in the crucible.

The crystallization furnace 22 has a tank 28 made of refractory material containing a bath 30 ofa material having a melting point less than that ofthe substance to be treated and able to raise to a temperature above the latter melting point while remaining in the liquid state and without suffering excessive vaporization losses. Tin, in particular, may be used without any special precautions up to a temperature of 1,400" C.- 1,500 C. Beyond that figure another :metal must be used, or a recovery condenser, which returns to the bath the tin that it collects, must be provided above the bath. The tank 28 may then be made of alumina, which is compatible with tin up to a temperature of l,500 C., which is enough for many manufactures, particularly that of stainless steel single-crystal blades for turboreactors or crude single crystals to be machined into shapes that do not permit direct casting.

A coil 32 for heating by high frequency induction is associated with the tank 28. This coil is supplied by a generator (not shown) having a regulating means modulating the power supplied in order to keep the bath at a constant temperature within a few degrees centigrade.

A cluster of moulds, designated as a whole by the reference numeral 34, receives in the molten state the substance from which the single crystals are to be prepared. The cluster 34 comprises a central channel 36 and. moulds 3B distributed round it and communicating by their bottoms with the base of the channel. These moulds terminate in a point at the top, so that one large nucleus may appear and gradually develop into one single crystal.

When, as in the case shown in the drawing, the form of the single crystals to be made is such that they cannot be extracted without destroying the moulds, the cluster is advantageously manufactured by moulding by the a cire perdue" method. The mould and channel are made, for example, of refractory cement, which is compatible with the molten tin and stainless steel at the temperatures contemplated.

The cluster is movable vertically from and t0 the bath by a mechanism comprising a stirrup 40 for grasping the top of the channel and a motorized lifting head 42 carried by the lid. This head is of conventional type and adapted to raise the stirrup M) with a uniform movement, without jolting it, ad

vantageously at an adjustable speed. The bottom position of the stirrup 40 is such, of course, that it then keeps the cluster 34 in a position in which it is completely immersed in the bath 30 and such that, by tilting the melting furnace 20, the liquid substance contained by the latter is poured into the channel The method for making single crystals, for example in the form of stainless steel blades, will become apparent from the preceding description. The melting furnace 20 is charged, and then the vessel is closed, and a vacuum of approximately 10 mm. Hg -is produced in it. The coils 26 and 32 are supplied with current to melt the steel in the crucible and the tin in the vessel 28 and bring the tin to a temperature slightly above the liquidus of the steel. The molten steel is then poured into the channel, which is kept in the bottom position in order that the moulds may be totally immersed in the tin. The vacuum is maintained during this operation to prevent the formation of bubbles by the trapping of gas, and subsequently, to prevent the oxidation of the steel and tin. The volume of steel poured must be such that the level in the channel 36 is above the tops of the moulds 38, in order that hydrostatic pressure may be exerted on the metal in the moulds. The heating of the vessel 22 is maintained and regulated so that the temperature of the tin is about 10 C. higher than the liquidus of the steel. The lifting head 42 is then actuated at a speed that is, for example, approximately 0.5 mm./min. but may vary in practice between l and l mm./min. The raising speed is essentially limited by the need to keep the crystallization isotherm at a level very slightly higher than that of the free surface of the bath, in

order that the isotherm may be flat.

As the moulds 38 emerge from the bath, crystals germinate at the top ends of the moulds and develop. The surface of the liquid practically represents the crystallization isotherm, and in these conditions the heat gradient and growth parameters are the same for all the moulds. The hydrostatic pressure at the interface of the crystal and molten substance due to the liquid in the channel prevents any separation and ensures exact dimensions. For this, of course, the channel 36 must be such that the solidification front in it is at a higher level than in the moulds 38. This being so, the resulting single crystals are free from grain joints and initiation of cleavage and do not undergo cavitation, diffusion, or intergranular brittleness.

The invention may, of course, be modified in many ways. In particular, tin may be replaced by other materials selected according to the melting point of the substance from which the single crystals are to be obtained. But the material should, like tin, have high conductivity. This ensures satisfactory flatness of the solidification isotherm, and therefore simultaneous growth of the single crystals in all the moulds, and a considerable heat gradient in the vertical direction from the free surface of the tin. A single furnace may be used, in which melting and crystallization take place in succession. But this is generally less advantageous than the system that has just been described.

We claim:

1. A method for the manufacture of single crystals of a substance, according to which the substance is melted, wherein the melted substance is subjected to hydrostatic pressure in a mould of high thermal conductivity closed at the top, the mould is immersed in a bath of molten material kept at a determined temperature higher than the melting point of the substance, and the single crystal is grown by gradually raising the mould from the bath to cause the growth of the single crystal from the top end of the mould downward.

2. A method as claimed in claim 1, for the simultaneous manufacture of several single crystals of the substance, according to which the molten substance is placed in a cluster of moulds whose bottom ends are connected to the same channel in which the level of the free surface of the substance is higher than the top points of the moulds. 

1. A method for the manufacture of single crystals of a substance, according to which the substance is melted, wherein the melted substance is subjected to hydrostatic pressure in a mould of high thermal conductivity closed at the top, the mould is immersed in a bath of molten material kept at a determined temperature higher than the melting point of the substance, and the single crystal is grown by gradually raising the mould from the bath to cause the growth of the single crystal from the top end of the mould downward.
 2. A method as claimed in claim l, for the simultaneous manufacture of several single crystals of the substance, according to which the molten substance is placed in a cluster of moulds whose bottom ends are connected to the same channel in which the level of the free surface of the substance is higher than the top points of the moulds. 